Ultra-thin metallic foil for lightning strike protection

ABSTRACT

A metallic foil for lightning strike protection in a composite aerospace structure having a length, a width, and a thickness of not more than 30 microns. There are a plurality of pores of a predefined geometric shape extending through the thickness of the metallic foil and being distributed across a surface area defined by the length and the width of the metallic foil. The plurality of pores in the aggregate define an open area of not more than 40% of the surface area and the metallic foil has a weight of not more than 115 g/m2. The metallic foil has a weight to conductivity ratio of not more than 0.40 gram-ohms per square.

FIELD OF THE INVENTION

This invention relates to a metallic foil used for lightning strikeprotection in composite aerospace structures and more particularly tosuch a metallic foil which is ultra-thin and light weight and yet hasvery low resistivity.

BACKGROUND OF THE INVENTION

Aluminum has been the principal material used in aircraft and aerospaceconstruction for the past 60 years. With the growing interest toconstruct more efficient aircraft, manufacturers are designing morecomponents out of light-weight composite materials. Current compositestructures include the fuselage, wing skins, engine nacelles, controlsurfaces, wing tips (winglets), and even rotary blades on helicoptersand wind turbines. Composites, however, are poor conductors ofelectrical current. Without proper protection, composite materials aresusceptible to severe damage in the event of a lightning strike. Todate, aircraft manufacturers have used aluminum or copper expanded foilsor woven wire mesh incorporated into the surface of these compositestructures to dissipate lightning strike energy and prevent damage tothe composite material.

Of the two approaches, expanded metal foils have become the industrystandard and are superior to woven wire as they do not unravel or haveloose strands that may become problematic during processing into apre-preg material or when conducting a dry lay-up as part of thecomposite manufacturing process. The homogenous design of expanded metalfoils also ensures uncompromised conductivity even when forming thematerial into a variety of shapes and contours and it provides a smoothsurface on the end product. Expanded metal foils used in thisapplication must be manufactured with tight tolerances to meet aspecific weight, open area, and conductivity requirements.

Aircraft manufactures use design guidelines, such as those set forth bySAE International in its Aerospace Recommended Practice (ARP) 5414,which defines lightning strike zones (areas of the aircraft moresusceptible to lightning strikes (e.g. Zone 1A, 1B). It also providesrequired electrical withstand capabilities for such strike zones. Forexample, often materials are required to have the ability to withstand aZone 1A strike of 200,000 amps. For expanded foils, due to thelimitations of the expansion process, the thinnest material possible tomeet this criteria to date has been produced using 42 micron foils. Theweight of this material is 175 grams per square meter, the resistivityis 3.6 milliohms per square, and the foil has 56% open area. A way tocharacterize the performance of foils in this application is to assessthe foil's weight to conductivity ratio, with conductivity being theinverse of resistivity and represented in gram-ohms per square. For theabove expanded foil, its weight to conductivity ratio is 0.63 gr-ohms.

Aircraft manufacturers are always looking for ways to increaseefficiency, reduce costs, improve fuel economy, and reduce the amount ofCO₂ emissions. One clear way to achieve these objectives is to reduceaircraft weight. By reducing the weight of the composite material, theoverall weight of the aircraft may be reduced; however, the conductivitycriteria required for specific strike zones per SAE ARP5414 must stillbe satisfied. Thus, it would be desirable to produce thinner and lighterfoils, which still meet the required electrical withstand capabilities.Another way of stating this is that it would be very desirable tominimize the foil weight to conductivity ratio.

SUMMARY OF THE INVENTION

The benefits and advantages of the present invention over existingsystems will be readily apparent from the Detailed Description tofollow. One skilled in the art will appreciate that the presentteachings can be practiced with embodiments other than those summarizedor disclosed below.

In one aspect the invention features a metallic foil for lightningstrike protection in a composite aerospace structure. The metallic foilcomprising copper or a copper alloy having a length, a width, athickness of not more than 30 microns. There are a plurality of pores ofa predefined geometric shape extending through the thickness of themetallic foil and being distributed across a surface area defined by thelength and the width of the metallic foil. The plurality of pores in theaggregate define an open area of not more than 40% of the surface area.The metallic foil has a weight of not more than 115 g/m² and a weight toconductivity ratio of not more than 0.40 gram-ohms per square.

In other aspects of the invention one or more of the following featuresmay be included. The predefined geometric shape of the plurality ofpores may be non-circular. The predefined geometric shape of theplurality of pores may be one or more of elliptical, diamond, oval,hexagonal, and square. The area of each of the predefined geometricshapes may be between 0.5×10⁻³ and 5.0×10⁻³ square inches. A strandwidth between the predefined geometric shapes may be not greater than0.050 inches. The metallic foil may have an isotropic resistance. Thetop surface and the bottom surface may include a coating of an inertmaterial that is resistant to tarnishing and oxidation. The top surfaceand the bottom surface may include a coating of silane to improveadhesion to other surfaces.

In yet another aspect, the invention features a metallic foil forlightning strike protection in a composite aerospace structure, whereinthe metallic foil comprises aluminum or an aluminum alloy having alength, a width, a thickness of not more than 30 microns. There are aplurality of pores of a predefined geometric shape extending through thethickness of the metallic foil and being distributed across a surfacearea defined by the length and the width of the metallic foil. Theplurality of pores in the aggregate define an open area of not more than40% of the surface area. The metallic foil has a weight of not more than35 g/m² and a weight to conductivity ratio of not more than 0.19gram-ohms per square.

In further aspects of the invention one or more of the followingfeatures may be included. The predefined geometric shape of theplurality of pores may be non-circular. The predefined geometric shapeof the plurality of pores may be one or more of elliptical, diamond,oval, hexagonal, and square. The area of each of the predefinedgeometric shapes may be between 0.5×10⁻³ and 5.0×10⁻³ square inches. Astrand width between the predefined geometric shapes may be not greaterthan 0.050 inches. The metallic foil may have an isotropic resistance.The top surface and the bottom surface may include a coating of an inertmaterial that is resistant to tarnishing and oxidation. The top surfaceand the bottom surface may include a coating of silane to improveadhesion to other surfaces.

In an additional aspect, the invention features a method of making ametallic foil for lightning strike protection in a composite aerospacestructure. The method includes providing a metallic foil comprising acopper or a copper alloy having a length, a width, and a thickness ofnot more than 30 microns. The method also includes forming in themetallic foil a plurality of pores of a predefined geometric shapeextending through the thickness of the metallic foil and beingdistributed across a surface area defined by the length and the width ofthe metallic foil. The plurality of pores in the aggregate define anopen area of not more than 40% of the surface area. The metallic foilhas a weight of not more than 115 g/m² and a weight to conductivityratio of not more than 0.40 gram-ohms per square.

In other aspects of the invention one or more of the following featuresmay be included. The predefined geometric shape of the plurality ofpores may be one or more of elliptical, diamond, oval, and square. Thearea of the predefined geometric shapes may be between 0.5×10⁻³ and5.0×10⁻³ square inches and wherein a strand width between the predefinedgeometric shapes is not greater than 0.050 inches. The step of formingmay include perforating the plurality of pores in the metallic foil. Thestep of perforating may be accomplished using a perforating die topierce the metallic foil. The step of perforating may be accomplishedusing a laser to cut or ablate the metallic foil.

In a further aspect, the invention features method of making a metallicfoil for lightning strike protection in a composite aerospace structure,including providing a metallic foil comprising an aluminum or analuminum alloy having a length, a width, and a thickness of not morethan 30 microns. The method includes forming in the metallic foil aplurality of pores of a predefined geometric shape extending through thethickness of the metallic foil and being distributed across a surfacearea defined by the length and the width of the metallic foil. Theplurality of pores in the aggregate define an open area of not more than30% of the surface area. The metallic foil has a weight of not more than35 g/m² and a weight to conductivity ratio of not more than 0.19gram-ohms per square.

In other aspects of the invention one or more of the following featuresmay be included. The predefined geometric shape of the plurality ofpores may be one or more of elliptical, diamond, oval, and square. Thearea of the predefined geometric shapes may be between 0.5×10⁻³ and5.0×10⁻³ square inches and wherein a strand width between the predefinedgeometric shapes may be not greater than 0.050 inches. The step offorming includes perforating the plurality of pores in the metallicfoil. The step of perforating may be accomplished using a perforatingdie to pierce the metallic foil. The step of perforating may beaccomplished using a laser to cut or ablate the metallic foil.

These and other features of the invention will be apparent from thefollowing detailed description and the accompanying figures.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached Figures, wherein:

FIG. 1 is a cross sectional view of a composite panel including ametallic foil according to an aspect of this invention;

FIG. 2A is a perspective view of a portion of a foil according to anaspect of the invention;

FIG. 2B is a perspective view of a portion of a prior art expanded foil;

FIG. 3 is a perspective view of a perforating machine for producing ametallic foil according to an aspect of this invention;

FIG. 4A is a plan view of a portion of a foil according to an aspect ofthe invention having elliptically-shaped pores;

FIG. 4B is a plan view of a portion of a foil according to an aspect ofthe invention having diamond-shaped pores;

FIG. 4C is a plan view of a portion of a foil according to an aspect ofthe invention having oval-shaped pores;

FIG. 5A is a plan view of a portion of a foil according to an aspect ofthe invention having circular-shaped pores spaced such that the foil hasisotropic resistance; and

FIG. 5B is a plan view of a portion of a foil according to an aspect ofthe invention having square-shaped pores spaced such that the foil hasisotropic resistance;

DETAILED DESCRIPTION OF THE INVENTION

The disclosure and the various features and advantageous details thereofare explained more fully with reference to the non-limiting embodimentsand examples that are described and/or illustrated in the accompanyingdrawings and detailed in the following description. It should be notedthat the features illustrated in the drawings are not necessarily drawnto scale, and features of one embodiment may be employed with otherembodiments as the skilled artisan would recognize, even if notexplicitly stated herein. Moreover, all listed values throughout thespecification are nominal values and are subject to normal manufacturingtolerances.

Descriptions of well-known components and processing techniques may beomitted so as to not unnecessarily obscure the embodiments of thedisclosure. The examples used herein are intended merely to facilitatean understanding of ways in which the disclosure may be practiced and tofurther enable those of skill in the art to practice the embodiments ofthe disclosure. Accordingly, the examples and embodiments herein shouldnot be construed as limiting the scope of the disclosure. Moreover, itis noted that like reference numerals represent similar parts throughoutthe several views of the drawings.

The metallic foil according to an aspect of the invention is designed tobe ultra-thin, very light weight, and to have very low resistivity. As aresult, very low weight to conductivity ratios may be achieved. Withthese characteristics the foil may be used to protect aircraft fromlightning strikes of a significant magnitude (e.g. Zone 1A strikes of200 amps or more), while allowing the aircraft manufacturer to produce alighter weight and more efficient aircraft. The metallic foil is alsoconfigured to provide optimal adhesive qualities so that it can belayered into and effectively adhered to composite structures. Thetypical types of metallic material which may be used are copper,aluminum, and alloys thereof.

For a copper or copper alloy foil herein, the thickness may be not morethan 30 microns. The foil may have a plurality of pores which, in theaggregate, define an open area of not more than 40% of the surface area.The weight of the foil may be not more than 115 g/m². The foil may havea resistance of not more than 3.5 milliohms per square. This yields aweight to conductivity ratio of 0.40 gr-ohms. Compared with the 42micron thick expanded copper foil described above, which has a 0.63gr-ohms weight to conductivity ratio, this is over a 35% improvement.With the foils and processing used herein, it is expected that copper orcopper alloy foils as thin as approximately 12 microns would beachievable with comparable weight to conductivity ratios.

For an aluminum or aluminum alloy foil herein, the thickness may be notmore than 30 microns. The foil may have a plurality of pores which, inthe aggregate, define an open area of not more than 40% of the surfacearea. The weight of the foil may be not more than 35 g/m². The foil mayhave a resistance of not more than 5.5 milliohms per square. This yieldsa weight to conductivity ratio of 0.19 gr-ohms. Compared with the 0.63gr-ohms of the expanded copper foil, this is over a 70% improvement inthe weight to conductivity ratio. With the foils and processing usedherein, it is expected that aluminum or aluminum alloy foils as thin asapproximately 12 microns would also be achievable with comparable weightto conductivity ratios.

Foil characteristics such as thickness, weight, and resistivity arereadily quantifiable; however, adhesion is not. For purposes of ourdescription, good adhesion of the foil to a composite panel after acuring cycle is one that prevents substantial peeling (e.g. less than25%) of the foil upon application of a high water jet device following,for example, Volvo STD1049.5134, with the following parameters:pressure, duration, distance, sweeping angle, sweeping frequency, watertemperature, maximum peeled surface. The values for these parameters maybe defined based of the requirements of the particular application.

To provide context, the metallic foil according to an aspect of theinvention is shown embedded in a cross-section of a composite panel 10,FIG. 1, which may be used to construct the fuselage of an aircraft, forexample. The composite panel 10 includes a honeycomb core 12 sandwichedbetween a first plurality of carbon plies 14 and second plurality ofcarbon plies 16. The plurality of carbon plies 14 are affixed to thebottom of honeycomb 12, using a bonding adhesive 13, such as epoxyresin, which plurality of plies 14 may be on the bag-side, i.e. the sideof the composite panel that is cured against the vacuum bag. Theplurality of carbon plies 16 are affixed to the top side of honeycomb 12also with adhesive 13. The metallic foil 18, according to thisinvention, may be affixed directly to carbon plies 16 or to an optionalisolation ply 17, interposed between metallic foil 18 and carbon plies16. Finally, a surfacing film 19 (e.g. an epoxy-based resin) may besecured to metallic foil 18 or, alternatively, foil 18 may be embeddedwithin surfacing film 19.

Referring to FIG. 2A, a perspective view of a portion 30 of metallicfoil 18 is shown. Portion 30 of foil 18 is shown to have a length, L,and a width, W, which define a surface area, A, of L*W. Distributedacross surface area A are a plurality of pores or openings 32, whichextend through the thickness, T, of the foil. As noted above, T may beultra-thin (i.e. 30 microns or less). For comparison, a portion 40 of anexpanded material having a thickness, TE, of approximately 42 microns,is depicted in FIG. 2B. As described above, this is the thinnestexpanded material currently manufactured. Expanded material is producedby slitting and stretching a material having a greater startingthickness to form a thinner material having a plurality ofpores/openings 42, which in this case are diamond-shaped. As notedabove, due to the limitations of the expansion process, this is thethinnest material achievable while still meeting the lowresistivity/high conductivity and strength requirements.

Thinner foils are achievable according to this invention; however, thepore size and spacing, as well as the amount of open area relative tototal foil area must be optimized to achieve desired weight, strength,adhesion capability, and conductivity. Portion 30 of foil 18 in thisexample has uniformly spaced elliptical-shaped pores, which may beformed in the foil by mechanically punching the material or using alaser to cut or ablate the material to form the pores. It is apparentfrom FIGS. 2A and 2B that the expanded material portion 40 has a greateramount of open area (defined by aggregate area of pores 42) relative tototal area, A_(E) (L_(E)*W_(E)), than does portion 30 of foil 18, whichhas less open area (defined by aggregate area of pores 32) relative tototal area, A. Limiting the open area allows the metallic foil 18 to bemade much thinner while still achieving the required strength andresistivity characteristics.

Before describing in more detail below how the open area in metallicfoil 18 may be formed with pores of different shapes, sizes, spacing andpatterns the manufacturing process for such metallic foils is described.

A method of manufacturing of the ultra-thin metallic foil according toan aspect of this invention may include an in-line process ofperforating the metallic foil with specifically sized and spaced holesof various geometric shapes to achieve desired performancecharacteristics. This may be accomplished using a perforating machine,such as machine 50 depicted in FIG. 3. The aerospace industry demandsthat these thin perforated foils be produced at widths exceeding 37inches, which is outside the process limits for typical perforatingequipment. Also, the foils must be substantially free of burrs, tearsand contaminants. This requires the use of precision perforating diesand equipment, which may be obtained commercially from reel-to-reelperforating machine builders.

A key element of the die-based perforation process is the use of abacking material, such as 40 lb Kraft paper, to “carry” the ultra-thinfoils through the perforating process. The use of the carrieressentially adds thickness to the material being perforated, whichprevents tearing and also helps to provide a precise, sharp cut withminimal burring. Referring again to FIG. 3, a wide web of metallic foil52 (of 37 inches or more) with backing material (not visible) is shownbeing fed under roller 54 and into the cutting dies 56. The backingmaterial also helps to control tension of the metal foil 52 as ittraverses through the cutting dies 56. While not shown in this view, theweb of solid foil 52 may be processed in a reel to reel operation. Inother words, solid metallic foil 52 with backing material may be fedinto machine 50 via a pay-out reel and the web of perforated metallicfoil may be received from machine and collected on a take-up reel.

An alternate approach to using a mechanical die-based perforatingmachine, such as machine 50, would be to use a reel-to-reelgalvanometric laser to accomplish the precision cutting/ablating tomanufacture the metallic coils according to an aspect of this invention.Although more costly, laser technologies have advanced to the pointwhere this process is an effective alternative to die-based perforating,especially since the material is thin and pore sizes are relativelysmall.

The metallic foil 52 may be pretreated to improve its durability andadhesion characteristics. In addition, the metallic foil may bepassivated, which creates an inert surface that is resistant totarnishing and oxidation. Also, the metallic foil may be coated withsilane, which helps to provide the material with improved adhesioncharacteristics.

By perforating the thin metallic foil 52, a plurality of openings of apredefined geometric shape extending through the thickness of themetallic foil and being distributed across a surface area may be formed.The aggregate amount of open area formed by the openings is configuredto provide maximum conductivity while maintaining as light a weight aspossible, which may be as light as 115 grams per square meter or less.Sizing of the openings/perforations is very important in order tomaintain desired strength, conductivity, and effective adhesionproperties and the following sections describe exemplary methods fordesigning the open area for metallic foils according to this invention.

Open Area

The open area of the metallic foil according to this invention generallyneeds to be below 40% of the overall surface area of the metallic foilin order to maintain sufficient strength and conductivity. Withoutsufficient strength, the foil will tear or break when incorporating itinto into a pre-preg or surfacing film and/or during manufacturing thecomposite laminate structure. Without enough foil material, the requiredamount of conductivity needed to absorb the significant electricalcurrent generated during a lightning strike will not be achieved. At thesame time, it is desired to maximize open area to minimize the weightand allow the laminate adhesives to effectively seep through theperforations and create “wet-out” uniformly to the outer surface of thestructure, releasing any air bubbles and creating a smooth, homogeneousstrong bond between the layers of the composite. In comparison, prior tothe current metallic foils, the lowest amount of open area in expandedfoils is approximately 55% open area, but thicker and heavier materialmust be used to provide the required strength and conductivity.

For proper adhesion, the pore area is optimal when it is between0.5×10⁻³ and 5.0×10⁻³ square inches. Historically, lightning strikeapplications have primarily utilized either a woven material, whichyields a square pattern, or expanded material, which yields a diamond orhexagonal shapes. By using perforated pores, different shapes may beused, including ovals, circles, and elliptical shapes. The mostefficient shapes, in terms of electric field lines, are rounded, assharp corners that are necessarily prevalent in woven or expandedmaterials create higher flux density in the areas of the sharp corners.These higher flux areas naturally increase the measured resistivity ofthe material.

Referring to FIG. 4A, a portion 60 a of a foil is shown to includeequally sized, elliptically-shaped pores 62 a, 64 a, 66 a, and 68 a. Thepores may be sized as required for a particular application as long asthe length, A, and width, B, provide an overall area of the poresbetween 0.5×10⁻³ and 5.0×10⁻³ square inches, as required to achieveproper adhesion. With pores of a larger size, spacing between pores (orstrand width, S) to obtain the desired conductivity tends to be toowide, thereby increasing the risk of adhesion difficulties. The strandwidth, S, is desired to be below 0.050 inches (1.25 mm) for properadhesion.

The open area of material may be determined by the following equation:

Open Area=1−Desired Weight/(Thickness×Density)  (1)

As an example, taking a desired weight of 100 grams per square meter(gsm) and using 17 micron thick copper as the desired material, whichhas a density of 8.89×10⁶ g/m³, the open area is determined to be:

Open Area=1−100/(17×10⁻⁶×8.89×10⁶)=33.8%  (2)

As noted above, open area of less than 40% of the overall surface areaof the metallic foil is required in order to maintain sufficientstrength and conductivity. Therefore, 17 micron thick copper materialhaving a weight of 100 gsm will be suitable to produce a metallic foilaccording to the invention.

Pore Spacing

Once the targeted open area, pore size and shape are selected, thespacing of the pores can be determined to achieve the resultingconfiguration. Carrying on with the example above, for an ellipticalshape pore, calculations are as follows:

Area of ellipse: pi*pore length(A)/2*pore height(B)/2  (3)

Open area=ellipse area*2/(X)*(Y)  (4)

With spacing between pores being equal in both X and Y directions:

X=SQRT((3.1416*A*B/2)/(open area))  (5)

Using as an example, an elliptical pore size of 0.060×090 inches, with adesired open area of 33.8%:

X=SQRT(3.1416×0.06 in×0.09 in/2/0.32)=0.158 in  (6)

With a pore spacing of 0.158 inches, this configuration measures astrand width (“S”) of approximately 0.049 inches or 31% of X/Y. Since0.050 inches is the largest targeted strand width, S, as notedpreviously, this pore size would approximately be the maximum sizerecommended for this material for use in a lightning strike application.

Similar configurations can be implemented with the above pore dimensionsusing different shapes, including diamonds and ovals. Referring to FIG.4B, a portion 60 b of a foil is shown to include equally sized,diamond-shaped pores 62 b, 64 b, 66 b, and 68 b, which may be used toform a 17 micron thick copper foil suitable for lightning strikeapplications. Referring to FIG. 4C, a portion 60 c of a foil is shown toinclude equally sized, oval-shaped pores 62 c, 64 c, 66 c, and 68 cwhich also may be used to form a 17 micron thick copper foil suitablefor lightning strike applications.

Using ovals, ellipses, or diamond shape pores, as shown in FIGS. 4A-4C,will produce very low resistivity foils, however, the resistivity willnot be equal in all direction, i.e. it will be “anisotropic”. Theresistivity along the “X” direction versus along the “Y” direction willdiffer depending upon the spacing and length to height ratio of thepores. It is sometimes desired to utilize a foil with conductiveproperties that are approximately equal in all directions, i.e.“isotropic”. To achieve isotropic features for conductivity, asymmetrical pattern may be used, either in the form of circles orsquares.

Referring to FIG. 5A, a portion 70 a of a foil is shown to includeequally sized, and symmetrically spaced circle-shaped pores 72 a, 74 a,76 a, and 78 a. Referring to FIG. 5B, a portion 70 b of a foil is shownto include equally sized, and symmetrically spaced square-shaped pores72 b, 74 b, 76 b, and 78 b. In both configurations, the foil may bedesigned to be isotropic in terms of resistance and also suitable forlightning strike applications by having the desired strength,conductivity, and effective adhesion properties described above.

The following is a table of exemplary configurations using 18 microncopper foil, including differing geometric shapes and sizes that havebeen determined to optimally meet adhesion, thickness, weight, andconductivity requirements according to this invention.

TABLE 1 18 micron Copper Spacing (in. x in.) Resistance Weight: MinOpening Along Along Open Width: Max Conductivity Opening Area WidthLength Area Length Resistance Weight Ratio Shape Size (in.) (in.²) (in.)(in.) (%) (mohms/sq) (mohms/sq) (grams) (gr-ohms) Circular .060 dia0.0028 0.175 0.100 32%   1:1 2 103 0.21 Elliptical .060 × .040 0.00190.110 0.110 31% 1.3:1 2.6 105 0.27 Elliptical .080 × .035 0.0022 0.1500.090 33% 1.8:1 3.4 103 0.35 Diamond .088 × .044 0.0019 0.110 0.110 32%1.4:1 2.8 103 0.29 Oval  .077 × 0.050 0.0032 0.144 0.144 31% 1.3:1 2.4106 0.25 Square .054 × .054 0.0029 0.126 0.126 37%   1:1 2.5 97 0.24

In Table 1 are included the specifications of the pores contained invarious 18 micron foils made of copper, including perforation shape,size (dimensions and area), and spacing. Also included are the amount ofopen area in each foil design, weight, maximum resistance, andresistance characteristics, i.e. isotropic or anisotropic. With theanisotropic foils the resistance ratio across the width of the foilrelative to the resistance along the length of the foil differs (e.g.1.3:1), while the isotropic foils have a ratio of 1:1.

The following is a table of exemplary configurations using 25 micronaluminum foil, including differing geometric shapes and sizes that havebeen determined to optimally meet adhesion, thickness, weight, andconductivity requirements according to this invention.

TABLE 2 25 micron Aluminum Spacing (in. x in.) Resistance Weight: MinOpening Along Along Open Width: Max Conductivity Opening Area WidthLength Area Length Resistance Weight Ratio Shape Size (in.) (in.²) (in.)(in.) (%) (mohms/sq) (mohms/sq) (grams) (gr-ohms) Circular .060 dia0.0028 0.175 0.100 32%   1:1 4.5 23 0.10 Elliptical .060 × .040 0.00190.110 0.110 31% 1.3:1 5.8 23 0.14 Diamond .088 × .044 0.0019 0.110 0.11032% 1.4:1 6.3 23 0.14 Oval  .077 × 0.050 0.0032 0.144 0.144 31% 1.3:15.4 23 0.12 Square .054 × .054 0.0029 0.126 0.126 37%   1:1 5.6 21 0.12

While we have described several specific examples of metallic foils,these are not intended to limit the invention and, using the designcriteria provided herein, many different configurations of foils may beproduced for various applications. Accordingly, many foil designsfalling within the bounds of the invention may be produced with verybeneficial attributes; namely, foils that are ultra-thin (i.e. 30microns or less), very light weight (i.e. 115 grams/m² or less), andvery low weight to conductivity ratios (i.e. 0.40 gram-ohms).Notwithstanding broad range of foil designs possible, the followinggeneral parameters for an optimized design are provided: opening sizearea of 0.5×10⁻³ to 5.0×10⁻³ square inches, open area between 25% and35%, and spacing maintained between 0.100 and 0.300 inches. Foils withthese criteria allow for very good adhesion and conductivity in bothdirections while also providing a very light weight foil.

It should be noted that some possible foil designs may have higherresistivity values, such as the 25 micron aluminum films in Table 2.These foils have resistivity values from 4.5-6.3 milliohms per square,which exceeds the maximum value of 3.5 milliohms per square for zone 1Alightning strike protection. However, these foils still have very lowweight to conductivity ratios and are very good candidates for lessintense lightning strike protection, e.g. Zone 2 lightning strikeprotection.

In comparison to the prior art expanded foil, the weight to conductivityratio is significantly improved, with the current foil while stillmaintaining good adhesion properties. This is due to the reduced foilthickness and specific pore geometry and spacing. For the 18 microncopper foil samples from Table 1, improvements in weight to conductivityratios of greater than 50-65% relative to the 42 micron expanded foildescribed above may be obtained. And, even greater improvements can beobtained with the aluminum foil. For the 25 micron aluminum foil samplesfrom Table 2, improvements in weight to conductivity ratios of greaterthan 75-80+% relative to the 42 micron expanded foil may be obtained

While the foregoing description of the invention enables one of ordinaryskill to make and use what is considered presently to be the best modethereof, those of ordinary skill will understand and appreciate theexistence of variations, combinations, and equivalents of the specificembodiments and examples herein. The above-described embodiments of thepresent invention are intended to be examples only. Alterations,modifications and variations may be effected to the particularembodiments by those of skill in the art without departing from thescope of the invention, which is defined solely by the claims appendedhereto. The invention is therefore not limited by the above describedembodiments and examples.

Having described the invention, and a preferred embodiment thereof, whatis claimed as new and secured by Letters Patent is:
 1. A metallic foilfor lightning strike protection in a composite aerospace structure, themetallic foil comprising copper or a copper alloy having a length, awidth, a thickness of not more than 30 microns, and a plurality of poresof a predefined geometric shape extending through the thickness of themetallic foil and being distributed across a surface area defined by thelength and the width of the metallic foil; wherein the plurality ofpores in the aggregate define an open area of not more than 40% of thesurface area; wherein the metallic foil has a weight of not more than115 g/m²; and wherein the metallic foil has a weight to conductivityratio of not more than 0.40 gram-ohms per square.
 2. The metallic foilof claim 1 wherein the predefined geometric shape of the plurality ofpores is non-circular.
 3. The metallic foil of claim 2 wherein thepredefined geometric shape of the plurality of pores is one or more ofelliptical, diamond, oval, hexagonal, and square.
 4. The metallic foilof claim 3 wherein the area of each of the predefined geometric shapesis between 0.5×10⁻³ and 5.0×10⁻³ square inches.
 5. The metallic foil ofclaim 4 wherein a strand width between the predefined geometric shapesis not greater than 0.050 inches.
 6. The metallic foil of claim 1wherein the metallic foil has an isotropic resistance.
 7. The metallicfoil of claim 1 wherein the top surface and the bottom surface include acoating of an inert material that is resistant to tarnishing andoxidation.
 8. The metallic foil of claim 7 wherein the top surface andthe bottom surface include a coating of silane to improve adhesion toother surfaces.
 9. A metallic foil for lightning strike protection in acomposite aerospace structure, the metallic foil comprising aluminum oran aluminum alloy having a length, a width, a thickness of not more than30 microns, and a plurality of pores of a predefined geometric shapeextending through the thickness of the metallic foil and beingdistributed across a surface area defined by the length and the width ofthe metallic foil; wherein the plurality of pores in the aggregatedefine an open area of not more than 40% of the surface area; whereinthe metallic foil has a weight of not more than 35 g/m²; and wherein themetallic foil has a weight to conductivity ratio of not more than 0.19gram-ohms per square.
 10. The metallic foil of claim 9 wherein thepredefined geometric shape of the plurality of pores is non-circular.11. The metallic foil of claim 10 wherein the predefined geometric shapeof the plurality of pores is one or more of elliptical, diamond, oval,hexagonal, and square.
 12. The metallic foil of claim 11 wherein thearea of each of the predefined geometric shapes is between 0.5×10⁻³ and5.0×10⁻³ square inches.
 13. The metallic foil of claim 12 wherein astrand width between the predefined geometric shapes is not greater than0.050 inches.
 14. The metallic foil of claim 9 wherein the metallic foilhas an isotropic resistance.
 15. The metallic foil of claim 9 whereinthe top surface and the bottom surface include a coating of an inertmaterial that is resistant to tarnishing and oxidation.
 16. The metallicfoil of claim 15 wherein the top surface and the bottom surface includea coating of silane to improve adhesion to other surfaces.
 17. A methodof making a metallic foil for lightning strike protection in a compositeaerospace structure, the method comprising: Providing a metallic foilcomprising a copper or a copper alloy having a length, a width, and athickness of not more than 30 microns; Forming in the metallic foil aplurality of pores of a predefined geometric shape extending through thethickness of the metallic foil and being distributed across a surfacearea defined by the length and the width of the metallic foil; whereinthe plurality of pores in the aggregate define an open area of not morethan 40% of the surface area; wherein the metallic foil has a weight ofnot more than 115 g/m²; and wherein the metallic foil has a weight toconductivity ratio of not more than 0.40 gram-ohms per square.
 18. Themethod of claim 17 wherein the predefined geometric shape of theplurality of pores is one or more of elliptical, diamond, oval, andsquare.
 19. The method of claim 18 wherein the area of the predefinedgeometric shapes is between 0.5×10⁻³ and 5.0×10⁻³ square inches andwherein a strand width between the predefined geometric shapes is notgreater than 0.050 inches.
 20. The method of claim 17 wherein the stepof forming includes perforating the plurality of pores in the metallicfoil.
 21. The method of claim 20 wherein the step of perforating isaccomplished using a perforating die to pierce the metallic foil. 22.The method of claim 21 wherein the step of perforating is accomplishedusing a laser to cut or ablate the metallic foil.
 23. A method of makinga metallic foil for lightning strike protection in a composite aerospacestructure, the method comprising: providing a metallic foil comprisingan aluminum or an aluminum alloy having a length, a width, and athickness of not more than 30 microns; forming in the metallic foil aplurality of pores of a predefined geometric shape extending through thethickness of the metallic foil and being distributed across a surfacearea defined by the length and the width of the metallic foil; whereinthe plurality of pores in the aggregate define an open area of not morethan 40% of the surface area; wherein the metallic foil has a weight ofnot more than 35 g/m²; and wherein the metallic foil has a weight toconductivity ratio of not more than 0.19 gram-ohms per square.
 24. Themethod of claim 23 wherein the predefined geometric shape of theplurality of pores is one or more of elliptical, diamond, oval, andsquare.
 25. The method of claim 24 wherein the area of the predefinedgeometric shapes is between 0.5×10⁻³ and 5.0×10⁻³ square inches andwherein a strand width between the predefined geometric shapes is notgreater than 0.050 inches.
 26. The method of claim 23 wherein the stepof forming includes perforating the plurality of pores in the metallicfoil.
 27. The method of claim 26 wherein the step of perforating isaccomplished using a perforating die to pierce the metallic foil. 28.The method of claim 26 wherein the step of perforating is accomplishedusing a laser to cut or ablate the metallic foil.